[CodeGen] Don't resolve the stack protector frame accesses until PEI

[CodeGen] Don't resolve the stack protector frame accesses until PEI

Currently, stack protector loads and stores are resolved during
LocalStackSlotAllocation (if the pass needs to run). When this is the
case, the base register assigned to the frame access is going to be one
of the vregs created during LocalStackSlotAllocation. This means that we
are keeping a pointer to the stack protector slot, and we're using this
pointer to load and store to it.

In case register pressure goes up, we may end up spilling this pointer
to the stack, which can be a security concern.

Instead, leave it to PEI to resolve the frame accesses. In order to do
that, we make all stack protector accesses go through frame index
operands, then PEI will resolve this using an offset from sp/fp/bp.

Differential Revision: https://reviews.llvm.org/D64759

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@367068 91177308-0d34-0410-b5e6-96231b3b80d8